qemu-img: assert block_job_get() does not return NULL in img_commit()

qemu-img: assert block_job_get() does not return NULL in img_commit()

Although the function block_job_get() can return NULL, it would be a
serious bug if it did so (because the job yields before executing anything
(if it started successfully); but otherwise, commit_active_start() would
have returned an error).  However, as a precaution, before dereferencing
the 'job' pointer in img_commit() assert it is not NULL.

Signed-off-by: Liam Merwick <Liam.Merwick@oracle.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1541453919-25973-4-git-send-email-Liam.Merwick@oracle.com
Signed-off-by: Max Reitz <mreitz@redhat.com>